if both of those are in the same part of the question ie both as a) then I would say that you do it by simultaneous equations
x + y = -8 --> leave
x - 2y = 7 --> multiply by -1
x + y = -8
-x + 2y = -7
add these
3y = -15 --> y = -5
x + -5 = 8 --> x = -3
x - 2(-5) = 7 --> x = -3
x = -3
y = -5
both equations work, so yeah I think this is the right way to do it